1. Field of the Invention
This invention relates to a drawing head comprising a base plate which has a central bore and which is nonrotatably mounted on a drawing machine, a straightedge support assembly which is mounted on a pin in the central bore which has a straightedge support with a nonrotatably attached top cover plate supporting a gripping knob, a spacing ring between the base plate and the cover plate which turns about the central axis of rotation, and, if necessary, base locking means for releasably retaining the spacing ring with respect to the base plate, whereby each locking means has a locking pawl which is pivotally positioned without play about an axis parallel to the central axis of rotation by means of a pivot jaw assembly and which locks by means of a locking tooth urged or pressed against a locking disc secured on the spacing ring.
2. Description of the Prior Art
One such drawing head is shown in German patent specification (Offenlegungschrift) No. 26 42 424.8, which has a locking means on the upper side for releasably locking the straightedge support assembly with respect to the spacing ring, as well as a locking means on the base or underside for releasably locking the spacing ring with respect to the base plate. The locking pawls are pivotally positioned without play about an axis parallel to the central axis by means of a pivot jaw assembly. The pivot jaw assembly is constructed such that the pivot pin is inserted into the pivot jaw axially from a bearing surface and the pivot jaw is constructed in a recess in the locking pawl. While the locking pawl is positioned in the plane of the adjoining locking disc, the pivot pin is anchored below and above this plane in the bearing surfaces of either the straightedge support assembly or the base plate. The substantial locking forces which operate on the locking means in the locked phase produce thereby undesirable stress moments perpendicular to the axis of rotation which interfere with the long-term precision of the drawing head.